Population dynamics and reproductive biology of the mysid Orientomysis japonica in Tango Bay, Japan

抄録

The mysids, Orientomysis spp., are common in shallow coastal waters and estuaries in Japan, and are reported to be one of the main prey items for juveniles of many commercially important fish species. Some studies have suggested that the seasonal dynamics of mysids are an important factor determining the biomass and production structure of demersal fish communities. In this study, seasonal variation in the population structure and reproductive traits of Orientomysis japonica Marukawa, 1928 were investigated in Tango Bay, the western part of Wakasa Bay, Japan, from April 2007 to April 2009. Mean densities at depths of 5 and 10 m were highest in spring (ca. 100 ind. m⁻²), decreasing drastically in early summer to nearly 0 ind. m⁻². Juveniles and immature individuals dominated the population (>80%) in spring and decreased in summer (<50%). Brooding females were captured year-round with mean body lengths ranging from 6.9 (August 2007) to 13.1 mm (February 2008). Body lengths of brooding females were negatively correlated with temperature. Brood size ranged from 1 to 102 and increased with female body length. The body sizes of all embryonic/larval stages within the marsupium were also negatively correlated with temperature. These results suggest that numerous large offspring released by the highly reproductive overwintering generation led to density increases and the maintenance of a high population density, while conversely, the summer–autumn generation exhibited repetitive short-term reproduction under high temperatures.